FBAR devices that incorporate one or more film bulk acoustic resonators (FBARs) form part of an ever-widening variety of electronic products, especially wireless products. For example, modern cellular telephones incorporate a duplexer in which each of the band-pass filters includes a ladder circuit in which each element of the ladder circuit is an FBAR. A duplexer incorporating FBARs is disclosed by Bradley et al. in U.S. Pat. No. 6,262,637 entitled Duplexer Incorporating Thin-film Bulk Acoustic Resonators (FBARs), assigned to the assignee of this disclosure and incorporated into this disclosure by reference. Such duplexer is composed of a transmitter band-pass filter connected in series between the output of the transmitter and the antenna and a receiver band-pass filter connected in series with 90° phase-shifter between the antenna and the input of the receiver. The center frequencies of the pass-bands of the transmitter band-pass filter and the receiver band-pass filter are offset from one another. Ladder filters based on FBARs are also used in other applications.
FIG. 1 shows an exemplary embodiment of an FBAR-based band-pass filter 10 suitable for use as the transmitter band-pass filter of a duplexer. The transmitter band-pass filter is composed of series FBARs 12 and shunt FBARs 14 connected in a ladder circuit. Series FBARs 12 have a higher resonant frequency than shunt FBARs 14.
FIG. 2 shows an exemplary embodiment 30 of an FBAR. FBAR 30 is composed a pair of electrodes 32 and 34 and a piezoelectric element 36 between the electrodes. The piezoelectric element and electrodes are suspended over a cavity 44 defined in a substrate 42. This way of suspending the FBAR allows the FBAR to resonate mechanically in response to an electrical signal applied between the electrodes.
Above-mentioned U.S. patent application Ser. No. 10/699,289, of which this application is a Continuation-in-Part discloses a band-pass filter that incorporates a decoupled stacked bulk acoustic resonator (DSBAR) composed of a lower FBAR, an upper FBAR stacked on lower FBAR and an acoustic decoupler between the FBARs. Each of the FBARs is composed of a pair of electrodes and a piezoelectric element between the electrodes. An electrical input signal is applied between electrodes of the lower FBAR and the upper FBAR provides a band-pass filtered electrical output signal between its electrodes. The electrical input signal may alternatively be applied between the electrodes of the upper FBAR, in which case, the electrical output signal is taken from the electrodes of the lower FBAR.
Above-mentioned U.S. patent application Ser. No. 10/699,481, of which this disclosure is a Continuation-in-Part, discloses a film acoustically-coupled transformer (FACT) composed of two decoupled stacked bulk acoustic resonators (DSBARs). A first electrical circuit interconnects the lower FBARs of the DSBARs in series or in parallel. A second electrical circuit interconnects the upper FBARs of the DSBARs in series or in parallel. Balanced or unbalanced FACT embodiments having impedance transformation ratios of 1:1 or 1:4 can be obtained, depending on the configurations of the electrical circuits. Such FACTs also provide galvanic isolation between the first electrical circuit and the second electrical circuit.
The FBAR described above with reference to FIG. 2 and devices, such as ladder filters, DSBARs and FACTs, incorporating one or more FBARs will be referred to generically in this disclosure as FBAR devices.
Currently, the FBAR stacks of thousands of FBAR devices are fabricated at one time on a wafer of silicon or another suitable material. Each FBAR device additionally comprises a portion of the wafer as its substrate. An FBAR stack is composed of layers of various materials in which at least one FBAR is defined. FBAR devices are typically packaged in a package described by Merchant et al. in U.S. Pat. No. 6,090,687 assigned to the assignee of this disclosure and incorporated by reference. The wafer on which the FBAR stacks are fabricated will be referred to as an FBAR wafer. Each FBAR stack fabricated on the FBAR wafer is surrounded by an annular gasket located on the surface of the FBAR wafer. A cap wafer is then placed adjacent the FBAR wafer and is bonded to the gaskets. The FBAR wafer, the cap wafer and the gaskets and FBARs between the wafers constitute a wafer stack. The wafer stack is then singulated into individual encapsulated FBAR devices, an exemplary one of which is shown in cross-sectional view in FIG. 3.
FIG. 3 shows an encapsulated FBAR device 50 composed of an FBAR device 52 and a package 54. FBAR device is composed of an FBAR stack 56 and a substrate 58. Substrate 58 also constitutes part of package 54. FBAR stack 56 is composed of layers of various materials in which at least one FBAR is defined. FBAR stack 56 is suspended over a cavity 60 defined in substrate 58. Substrate 58 was part of the above-mentioned FBAR wafer prior to singulation. FBAR stack 56 is surrounded by an annular gasket 62 bonded to the major surface of substrate 58. Cap 64, which was part of the above-mentioned cap wafer prior to singulation, is bonded to gasket 62 opposite substrate 58. Substrate 58, gasket 62 and cap 64 collectively define a hermetically-sealed chamber 66 in which FBAR stack 56 is located.
As noted above, FBAR stack 56 is suspended over cavity 60 defined in substrate 58. The large mismatch between the acoustic impedances of the materials of FBAR stack 56 (typically tens of megarayleighs (Mrayl)) and the air or other gas in cavity 60 (about 1 kilorayleigh (krayl)) acoustically isolates FBAR stack 56 from substrate 58. Similarly, the top surface 68 of FBAR stack 56 remote from substrate 58 is separated from cap 64 by a gap 70. Gap 70 is typically filled with air or another gas. The large mismatch between the acoustic impedances of the materials of FBAR stack 56 and the air or other gas in gap 70 acoustically isolates FBAR stack 56 from cap 64. Thus, FBAR stack 56 is acoustically decoupled from both substrate 58 and cap 64 and is therefore free to resonate mechanically in response to an electrical signal applied between its electrodes.
While the package 54 of the encapsulated FBAR device 50 shown in FIG. 3 is relatively simple and inexpensive, simpler and less expensive packaging is available. One example of such packaging involves encapsulating the FBAR stack in an encapsulant (not shown) that covers the FBAR stack and part of the substrate. However, mechanical contact between the encapsulant and the top surface 68 of the FBAR stack remote from the substrate degrades the electrical properties of the encapsulated FBAR device because the FBAR stack is no longer free to resonate mechanically.
What is needed, therefore, is an encapsulated FBAR device in which the FBAR stack is effectively acoustically isolated from the encapsulant.